Plasmodium infections are responsible for ~250 million clinical cases of malaria and nearly one million deaths each year worldwide. Plasmodium parasites are transmitted by anopheline mosquitoes, of which An. gambiae is the major vector in Africa. Although progress has been made towards the development of malaria vaccines, current strategies targeting liver and/or blood stage Plasmodium parasites will not stop disease transmission. Indeed, the long-term goal of malaria eradication will not be possible unless transmission is also blocked. To date, only one candidate transmission blocking vaccine (TBV) antigen (Pfs25) has reached clinical trial. However, Pfs25 cannot induce cross-species blocking (e.g. P. falciparum and P. vivax), and it cannot completely block malaria transmission. Thus, identification of additional TBV antigens that can prevent transmission of multiple clinically relevant species of Plasmodium and increase blocking efficacy remains a priority. The goals of this proposal are to evaluate a novel, highly conserved mosquito-expressed TBV vaccine candidate antigen and to use this molecule to identify additional TBV candidate antigens expressed by Plasmodium parasites. Recently, we reported that fibrinogen-related protein-1 (FREP1) expressed in An. gambiae is necessary for Plasmodium parasite infection in mosquitoes. New preliminary data show that FREP1 protein binds both rodent P. berghei and human P. falciparum infected red blood cells, suggesting that FREP1 is part of a highly conserved pathway of Plasmodium invasion in mosquitoes. Importantly, anti-FREP1 antibodies block the development of P. falciparum in mosquito membrane-feeding assays, illustrating the critical biological role of FREP1 during mosquito infection. We hypothesize that FREP1 expressed in An. gambiae and its binding partner(s) expressed in Plasmodium parasites can serve as novel, highly conserved TBV candidate antigens that limit Plasmodium infection in mammalian hosts and block transmission to the mosquito. To test this hypothesis, we have assembled a consortium of experts to identify the FREP1 binding partner(s) (FBPs) in Plasmodium parasites (Aim 1) and analyze the capacity of recombinant FREP1 and FBPs to serve as vaccine candidate antigens that limit Plasmodium infection in the mammalian hosts and block parasite development in mosquitoes (Aim 2). Successful completion of the proposed studies will determine novel vaccine targets for malaria control and reveal critical pathways of Plasmodium invasion in mosquito midguts.